Shell and tube type heat exchangers are widely used in a variety of industries in various applications for the heating or cooling of fluids. Basically a shell and tube type heat exchanger consists of a plurality of generally parallel tubes arranged in a bundle and placed within a shell. One fluid is circulated through the tubes while a second fluid circulates within the shell, over and around the tubes to establish heat transfer between the two fluids. End plates or other means are provided to physically keep the fluids separate, and appropriate connections are provided for conveying the fluids to and form the heat exchanger. Baffles are usually provided to direct the flow within the shell in several passes across the tubes to increase heat transfer. The baffles generally comprise plates that extend partially, but not completely, across the inside of the shell, with tubes passing through clearance holes in the baffles. The baffles perform the function of supporting the tubes at one or more intermediate points in their span from one end of the heat exchanger to the other, as well as serving to direct the flow of fluid within the shell in the desired path back and forth across the tubes.
It has long been recognized that care must be taken in the design and construction of heat exchangers to avoid the potentially damaging effects of vibration. Such vibration can be induced by flow conditions within the heat exchangers, or can be caused by vibration sources external to the heat exchanger, or a combination of both. Whatever the cause of the vibration, the greatest difficulty occurs when the frequency of vibrations coincides with, or falls within the same range as, the natural resonant vibration frequency of the tubes. If the internally induced or externally excited vibrations approach too closely to the resonant frequency of the tubes, severe vibration and damage can result. It is therefore necessary in the design of such heat exchangers to separate the natural frequency of the tubes by a safe margin from the frequency of the vibration to which the heat exchanger will be subjected during use.
In prior art multiple span baffled shell and tube heat exchanger the tubes are supported by the baffles, but are not entirely fixed. In order to facilitate manufacture and assembly, the holes in the baffle are drilled slightly larger than the outside diameter of tubes. The resulting structure allows a small amount of lateral movement of the tubes within the baffles. This arrangement of tubes and baffles may be mathematically modeled as beams simply supported at each intermediate baffle. The natural frequency of the tubes calculated in this manner is very close to the measured value.
In order to alter the natural frequency of the tubes for a particular heat exchanger design application, it may be possible to change the number or spacing of the baffles thereby to change the unsupported tube span. However, it may not be possible to change the tube span and baffle position due to heat transfer or pressure drop design requirements for the heat exchanger.
The present invention provides a method for fixing, as opposed to simply supporting, the tubes at the baffles, which increases the natural frequency of the tubes by a considerable amount without affecting other design and performance criteria for the heat exchanger. At the same time the fixing of the tubes at the baffles eliminates the small clearance between baffle and tube existing in prior heat exchangers referred to above, and eliminates impact damage between the tubes and baffles in the event that vibration problems do occur.